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Improving a Geophone to Produce an Affordable, Broadband Seismometer
Aaron Barzilai
PhD DefenseMechanical Engineering
Stanford University
January 25, 2000
2Aaron Barzilai Stanford University
Talk Outline
• Introduction
• Why isn’t a geophone an affordable, broadband seismometer?• How can a geophone be transformed into an affordable,
broadband seismometer? • How well do prototypes of a modified geophone perform?
3Aaron Barzilai Stanford University
The Primary Application
• A seismometer is a sensor that measures ground motion• PEPP (Princeton Earth Physics Project) is a group of
geophysics professors interested in placing seismometers in high school science classrooms
• Most areas don’t have local seismicity, so the only interesting signals are caused by teleseisms – large, distant earthquakes
Recorded at Stanford with a Guralp CMG-40T after a M7.3 Quake near Australia on 11/26/99
10 min
4Aaron Barzilai Stanford University
Teleseism Detection• Only low frequency (below 100 mHz-1 Hz) signals can
travel large distances
Requirements according to PEPP• Measure signals with periods as large as 30 sec
(frequencies as small as 33 mHz)• Constant sensitivity to ground velocity• Cost less than $1000
54 56 58 60 62 64 66-60-30
03060
Time [min]Gro
und
Vel
ocity
[um
/s]
5Aaron Barzilai Stanford University
Existing Commercial Seismometers
5 Hz –500 Hz
30 mHz –50 Hz
8 mHz –50 Hz
Constant Ground Velocity
Sensitivity
CostLow
Frequency Resolution
CommentType
$50PoorCheapestConventional
Geophone
$10,000GreatCompetitorGuralp
$20,000BestBestStreckeisen
6Aaron Barzilai Stanford University
Existing Commercial Seismometers
30 mHz–50 Hz
$1,000DecentNewModified Geophone
5 Hz –500 Hz
30 mHz –50 Hz
8 mHz –50 Hz
Constant Ground Velocity
Sensitivity
CostLow
Frequency Resolution
CommentType
$50PoorCheapestConventional
Geophone
$10,000GreatCompetitorGuralp
$20,000BestBestStreckeisen
7Aaron Barzilai Stanford University
Existing Commercial Seismometers
30 mHz–50 Hz
$1,0005x10-6NewModified Geophone
5 Hz –500 Hz
30 mHz –50 Hz
8 mHz –50 Hz
Constant Ground Velocity
Sensitivity
CostResolution at 30 mHz[(m/s)/√Hz]
CommentType
$502x10-4CheapestConventional
Geophone
$10,0008x10-8CompetitorGuralp
$20,0001x10-10BestStreckeisen
8Aaron Barzilai Stanford University
Introduction Recap
• Major application – Education• No commercially available seismometer delivers the
needed performance at a reasonable cost.• Plan to start with a geophone and improve it without
making cost unreasonable.
9Aaron Barzilai Stanford University
Talk Outline
• Introduction
• Why isn’t a geophone an affordable, broadband seismometer?
• How can a geophone be transformed into an affordable, broadband seismometer?
• How well do prototypes of a modified geophone perform?
10Aaron Barzilai Stanford University
What is a Conventional Geophone?
11Aaron Barzilai Stanford University
What is a Conventional Geophone?
CylinderCoil
Leaf SpringMagnet
12Aaron Barzilai Stanford University
How does a Conventional Geophone Work?
Cylinder
Coil
Leaf SpringMagnet
13Aaron Barzilai Stanford University
How does a Conventional Geophone Work?
14Aaron Barzilai Stanford University
How does a Conventional Geophone Work?
GroundMotion
15Aaron Barzilai Stanford University
How does a Conventional Geophone Work?
MechanicalSystem
GroundMotion
RelativeMotion
16Aaron Barzilai Stanford University
How does a Conventional Geophone Work?
MechanicalSystem
ElectricalSystem
GroundMotion
RelativeMotion
OutputSignal
17Aaron Barzilai Stanford University
Conventional Geophone Mechanical Sensitivity
MechanicalSystem
ElectricalSystem
kmb
mk
n
2=
=
ζ
ω22 2
1
nnssxy
ωζω ++=
&&Groundmotion
Relativemotion
Constant sensitivity below the resonant frequency
2nd Order System
10-6
10-5
10-4
10-3
10-2
10-2 10-1 100 101 102Mec
hani
cal S
ensi
tivity
[m/(
m/s
2 )]
Frequency [Hz]
18Aaron Barzilai Stanford University
100
101
102
103
104
105
10-2 10-1 100 101 102Ele
ctri
cal S
ensi
tivity
[V/m
]
Frequency [Hz]
Conventional Geophone Electrical Sensitivity
MechanicalSystem
ElectricalSystem
Reduced sensitivity at lower frequencies because measuring relative velocity
Faraday’s Law
yGt
V &=∂∂−= φ
Relativemotion
OutputVoltage Transduction Constant
[V/(m/s)] or [N/A]
19Aaron Barzilai Stanford University
Conventional Geophone Total Sensitivity
10-4
10-3
10-2
10-1
100
101
102
10-2 10-1 100 101 102Tot
al S
ensi
tivity
[V/(
m/s
)]
Frequency [Hz]
10-4
10-3
10-2
10-1
100
101
102
10-2 10-1 100 101 102Tot
al S
ensi
tivity
[V/(
m/s
2 )]
Frequency [Hz]
Electrical system measures velocity
Mechanical system has resonance
20Aaron Barzilai Stanford University
Conventional Geophone Resolution
Noise Sources• Thermomechanical noise• Circuitry noise
Geophone at low frequencies: Sensitivity ⇓, Resolution ⇑
][ySensitivit][ Noise
][ Resolution)/(
HzHz
)/(
smV
Vsm =
Want resolution to be a small number
21Aaron Barzilai Stanford University
Conventional Geophone Resolution
Usher et al., “A miniature wideband horizontal component feedback seismometer,” Background Signals,” Journal of Physics E:Scientific Instruments, Dec. 1977, vol.10, no.12
( )
Hz)/(
10
min
Hz)/(10
min
min
2105.5
105.5
mTb4
2
sm
sm
b
fx
x
fkx
π
−
−
×=
×=
∆=
&
&&
&&
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-2 10-1 100 101 102
Geo
phon
e R
esol
utio
n [(
m/s
)/√H
z]
Frequency [Hz]
Thermo-mechanical
Limit
22Aaron Barzilai Stanford University
Conventional Geophone Resolution
Rodgers, P.W., “Frequency Limits For Seismometers As Determined From Signal To Noise Ratios.Part 1. The Electromagnetic Seismometer,” BSSA, Apr. 1992, vol.82, no.2.
Geo100Ω
10kΩ
VO
Hz)/(
10
min 2105.5 sm
fx
π
−×=&
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-2 10-1 100 101 102
Geo
phon
e R
esol
utio
n [(
m/s
)/√H
z]
Frequency [Hz]
PredictedCircuitry Limit
Thermo-mechanical
Limit
23Aaron Barzilai Stanford University
Conventional Geophone Resolution
Barzilai et al.., “Technique for measurement of the noise of a sensor in the presence of large background signals,” Review of Scientific Instruments, July 1998, vol.69, no.7.
Geo100Ω
10kΩ
VO
Hz)/(
10
min 2105.5 sm
fx
π
−×=&
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-2 10-1 100 101 102
Geo
phon
e R
esol
utio
n [(
m/s
)/√H
z]
Frequency [Hz]
PredictedCircuitry Limit
MeasuredResolution
Thermo-mechanical
Limit
24Aaron Barzilai Stanford University
Talk Outline• Introduction• Why isn’t a geophone an affordable, broadband seismometer?
– What is a geophone?– How does a geophone work?– What is the sensitivity of a geophone?– What is the resolution of a geophone?
– How was the geophone’s resolution measured?
• How can a geophone be transformed into an affordable, broadband seismometer?
• How well do prototypes of a modified geophone perform?
25Aaron Barzilai Stanford University
Measuring a Conventional Geophone’s Resolution
Typical Experiment
10-9
10-8
10-7
10-6
10-5
10-1 100 101 102Gro
und
Vel
ocity
[(m
/s)/√
Hz]
Frequency [Hz]
26Aaron Barzilai Stanford University
Measuring a Conventional Geophone’s Resolution
Better Experiment
10-9
10-8
10-7
10-6
10-5
10-1 100 101 102
Frequency [Hz]
Geophone AGeophone B
Gro
und
Vel
ocity
[(m
/s)/√
Hz]
27Aaron Barzilai Stanford University
Determining Resolution from “Identical” Outputs
28Aaron Barzilai Stanford University
Determining Resolution from “Identical” Outputs
Coherence – – the fraction of the power of signal X that also appears in signal Y
Bendat and Piersol, Engineering Applications of Correlation and Spectral Analysis , 2nd ed. (Wiley, New York, 1993)
2XYγ
29Aaron Barzilai Stanford University
Determining Resolution from “Identical” Outputs
Coherence – – the fraction of the power of signal X that also appears in signal Y
( )YX
XYXY PSDPSD
CSD 22 )( =ωγ
Coherence
Bendat and Piersol, Engineering Applications of Correlation and Spectral Analysis , 2nd ed. (Wiley, New York, 1993)
2XYγ
Power SpectralDensity ∑
=
=dn
kk
dX T
TnG
1
2),(X2
)( ωω
Cross SpectralDensity ∑
=
=dn
kdXY TT
TnG
1k
*k ),(Y),(X2)( ωωω
)( Hz)/( 2sm
30Aaron Barzilai Stanford University
Determining Resolution from “Identical” Outputs
22
)()(
)()(
21
1)(
++
=
ωω
ωω
ωγ
U
N
U
N
XY
GG
GG
1)()(
)()(
21 ==
=
ωω
ωω
HH
GG NM
SeismicSignal
N
M
UX
Y
)(1 ωH
)(2 ωH
SensorNoise
TransferFunction
Output
+
+
+
+
31Aaron Barzilai Stanford University
Determining Resolution from “Identical” Outputs
22
)()(
)()(
21
1)(
++
=
ωω
ωω
ωγ
U
N
U
N
XY
GG
GG
)()()( ωωω UNX GGG +=
( ))(1)()(
)(1)()(2
2
ωγωω
ωγωω
XYXN
XYXN
NSDNSD
GG
−=
−=
Hz)(
Hz)( 2
sm
sm
SeismicSignal
N
M
UX
Y
)(1 ωH
)(2 ωH
SensorNoise
TransferFunction
Output
+
+
+
+
32Aaron Barzilai Stanford University
Determining Resolution from “Identical” Outputs
0
0.2
0.4
0.6
0.8
1
10-1 100 101 102
Coh
eren
ce []
Frequency [Hz]
10-10
10-9
10-8
10-7
10-6
10-5
10-1 100 101 102Gro
und
Vel
ocity
[(m
/s)/√
Hz]
Frequency [Hz]
Geophone AGeophone B
GeophoneResolution
2XYγ
33Aaron Barzilai Stanford University
Conventional Geophone Recap
• A conventional geophone has poor sensitivity at low frequencies because it inductively measures proof mass velocity
• The geophone’s poor low frequency sensitivity leads to poor low frequency resolution
• A technique has been demonstrated to measure the resolution of a seismometer in a noisy environment
34Aaron Barzilai Stanford University
Talk Outline
• Introduction• Why isn’t a geophone an affordable, broadband seismometer?
• How can a geophone be transformed into an affordable, broadband seismometer?
• How well do prototypes of a modified geophone perform?
35Aaron Barzilai Stanford University
How Can We Improve a Geophone?
Rather than measure proof mass velocity,
100101102103104105106107
10-2 10-1 100 101 102Ele
ctri
cal S
ensi
tivity
[V/m
]
Frequency [Hz]
36Aaron Barzilai Stanford University
How Can We Improve a Geophone?
Rather than measure proof mass velocity, measure proof mass displacement
Have obtained the best results measuringproof mass displacement capacitively
100101102103104105106107
10-2 10-1 100 101 102Ele
ctri
cal S
ensi
tivity
[V/m
]
Frequency [Hz]
37Aaron Barzilai Stanford University
Capacitive Geophone Mechanical Hardware
Moving Electrode
Fixed Electrodes
Fixed Electrodes
38Aaron Barzilai Stanford University
Capacitance Electrical System
Modulate Measure
Demodulator
Demodulate
VPOSN-1
Position Measurement
39Aaron Barzilai Stanford University
Capacitance Electrical System
Modulate Measure
Demodulator
Demodulate
VPOSN
Controller DriveCircuitry
Position Measurement
-1
Feedback
40Aaron Barzilai Stanford University
Capacitance Electrical System
Modulate Measure
Demodulator
Demodulate
VPOSN
Controller DriveCircuitry
SignalProcessing
VelocityOutput
Acceleration Output
Feedback
Position Measurement
-1
41Aaron Barzilai Stanford University
Capacitive Geophone Control Design
totalx&&
)( 2sm
Mechanical System
1200421
2 ++ ss
y
(m)
5106×posnV
(V)
CapacitivePosition Sensing
( )sKcontV
(V)
Controller
3741contI
(A)
Resistor
30contF
(N)
Coil-Magnetas Actuator(G)
025.1
Mass
inx&&
)( 2sm
Drive Circuitry
contx&&
)( 2sm
+
-
42Aaron Barzilai Stanford University
Capacitive Geophone Control Design
• Controller aims to keep capacitances equal
• Design controller to produce correct sensitivity transfer function– Apply forces to counteract gravity– Apply forces to counteract seismic input
• Ensure system is stable– Loop gain must be less than 1 when phase dips below -180 degrees– Loop gain: Product of the transfer functions of all blocks of the system
43Aaron Barzilai Stanford University
10-3 10-2 10-1 100 101 102 103-200
-150
-100
-50
0
50
Frequency [Hz]
Loop
Gai
nPh
ase
[deg
]
10-3 10-2 10-1 100 101 102 10310-210-1100101102103104
Frequency [Hz]
Loo
p G
ain
Mag
nitu
de []
Loop Gain With Unity Gain Controller
-180
Gain>1
44Aaron Barzilai Stanford University
Inner Controller: Low Pass Filter
inx&& totalx&&
)( 2sm
Mechanical System
1200421
2 ++ ss
y
(m)
contx&&
)( 2sm
5106×posnV
(V)
CapacitivePosition Sensing
125001
+s
contV
(V)
Controller
3741contI
(A)
Resistor
30contF
(N)Coil-Magnet
as Actuator(G)
025.1
Mass
2.5 M1000 µF
374
G
+
-
45Aaron Barzilai Stanford University
10-3 10-2 10-1 100 101 102 103-300-250-200-150-100
-500
Frequency [Hz]
Loop
Gai
nPh
ase
[deg
]
10-3 10-2 10-1 100 101 102 10310-8
10-6
10-4
10-2
100
102
Frequency [Hz]
Loo
p G
ain
Mag
nitu
de []
Loop Gain with Low Pass Filter
-180
Gain <1
46Aaron Barzilai Stanford University
Transfer Function with Low Pass Filter
10-3 10-2 10-1 100 101 102 10310-2
10-1
100
101
102
103
Frequency [Hz]Tra
nsfe
r Fun
ctio
n
Mag
nitu
de [V
/(m
/s2 )]
10-3 10-2 10-1 100 101 102 103-200-150-100
-500
50100
Frequency [Hz]
Tra
nsfe
r Fun
ctio
nPh
ase
[deg
]
47Aaron Barzilai Stanford University
Outer Controller: Lead Circuit
inx&& totalx&&
)( 2sm
Mechanical System
1200421
2 ++ ss
y
(m)
contx&&
)( 2sm
5106×posnV
(V)
CapacitivePosition Sensing
125001
+s
contV
(V)
Controller
3741contI
(A)
Resistor
30contF
(N)Coil-Magnet
as Actuator(G)
025.1
Mass
contx&&
)( 2sm 1
121
22
11
1
2
++
sCRsCR
RRcontV
(V)3741contI
(A)30
contF
(N)025.1
+
- -
48Aaron Barzilai Stanford University
Lead Circuit Transfer Function
10-3 10-2 10-1 100 101 102 10310-2
10-1
100
Frequency [Hz]
Lea
d C
ircui
tM
agni
tude
[]
10-3 10-2 10-1 100 101 102 103-100
102030405060
Frequency [Hz]
Lea
d C
ircu
itPh
ase
[deg
]
49Aaron Barzilai Stanford University
10-3 10-2 10-1 100 101 102 10310-2
10-1
100
101
102
Frequency [Hz]
Loo
p G
ain
Mag
nitu
de []
10-3 10-2 10-1 100 101 102 103-200-150-100
-500
50100
Frequency [Hz]
Loop
Gai
nPh
ase
[deg
]Loop Gain with Full Controller
Phase > -180
Gain =1
50Aaron Barzilai Stanford University
Transfer Function with Full Controller
10-3 10-2 10-1 100 101 102 10310-2
10-1
100
101
Frequency [Hz]Tra
nsfe
r Fun
ctio
n
Mag
nitu
de [V
/(m
/s2 )]
10-3 10-2 10-1 100 101 102 103-200-150-100
-500
50100
Frequency [Hz]
Tra
nsfe
r Fun
ctio
nPh
ase
[deg
]
51Aaron Barzilai Stanford University
Capacitance Electrical System
Modulate Measure
Demodulator
Demodulate
VPOSN
Controller DriveCircuitry
SignalProcessing
VelocityOutput
Acceleration Output
Feedback
Position Measurement
-1
52Aaron Barzilai Stanford University
Capacitive Geophone Predicted Sensitivity
• Design predicts constant sensitivity to ground velocity from 10 mHz – 50 Hz
10-1
100
101
102
10-3 10-2 10-1 100 101 102
Sens
itivi
ty [V
/(m
/s)]
Frequency [Hz]
53Aaron Barzilai Stanford University
Modified Geophone Design Recap
• Capacitively measuring proof mass position improves the sensitivity, and therefore resolution, of a geophone
• A capacitive geophone operates with feedback to tune it’s frequency response
• Feedback forces are applied by running current through the geophone’s coil
54Aaron Barzilai Stanford University
Talk Outline
• Introduction• Why isn’t a geophone an affordable, broadband seismometer?• How can a geophone be transformed into an affordable,
broadband seismometer?
• How well do prototypes of a capacitive geophone perform?
55Aaron Barzilai Stanford University
Capacitive Geophone Sensitivity
• Measurement roughly, but not exactly, matches prediction
• Results obtained by measuring ambient seismic signal with both Capacitive Geophone and Guralp
• Guralp’s measurement not quite accurate over 50 Hz10-1
100
101
102
10-3 10-2 10-1 100 101 102
Sens
itivi
ty [V
/(m
/s)]
Frequency [Hz]
56Aaron Barzilai Stanford University
Measured Ambient Seismic Signals
• Above 0.3 Hz, seismometers have virtually identical outputs
• Below 0.3 Hz, capacitive geophone noise is greater than the ambient seismic signal, except peak at 7 seconds caused by the tides.
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-3 10-2 10-1 100 101
Frequency [Hz]
Guralp CMG-40T
Gro
und
Vel
ocity
[(m
/s)/√
Hz]
CapacitiveGeophone
57Aaron Barzilai Stanford University
Seismometer Resolution
• Used coherence to determine capacitive geophone resolution above 0.3 Hz
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-3 10-2 10-1 100 101
Frequency [Hz]
Res
olut
ion
[(m
/s)/√
Hz]
CapacitiveGeophone
58Aaron Barzilai Stanford University
Seismometer Resolution
• Used coherence to determine capacitive geophone resolution above 0.3 Hz
• Capacitive Geophone has better resolution than a Conventional Geophone at low frequencies, but not at high frequencies
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-3 10-2 10-1 100 101
Frequency [Hz]
Res
olut
ion
[(m
/s)/√
Hz] Conventional
Geophone
CapacitiveGeophone
59Aaron Barzilai Stanford University
Seismometer Resolution
• Used coherence to determine capacitive geophone resolution above 0.3 Hz
• Capacitive Geophone has better resolution than a Conventional Geophone at low frequencies, but not at high frequencies
• Capacitive geophone is almost 2 orders of magnitude noisier than a Guralp CMG-40T
• Expect it can be improved…
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-3 10-2 10-1 100 101
Frequency [Hz]
Guralp CMG-40T
Res
olut
ion
[(m
/s)/√
Hz]
GeophoneThermo-
mechanicalLimit
ConventionalGeophone
CapacitiveGeophone
60Aaron Barzilai Stanford University
Seismic Vault
61Aaron Barzilai Stanford University
Seismic Vault
62Aaron Barzilai Stanford University
Seismic Vault: Uncovered
63Aaron Barzilai Stanford University
10-8
10-7
10-6
10-5
10-4
10-3 10-2 10-1 100 101
Frequency [Hz]
Res
olut
ion
[(m
/s)/√
Hz]
In Air
Inside"SeismicVault"
Environmental Coupling
30x10-6
6x10-6• At 30 mHz, seismic noise
reduces vault by a factor of 5
• Proves that outside seismic vault, low frequency resolution set by environmental noise
• Expect that is still true inside
• Expect vault to reduce temperature fluctuations and air currents
64Aaron Barzilai Stanford University
More on Environmental Coupling“…the sensor must be protected from variations of the atmospheric air pressure; its seismic mass would otherwise experience a variable buoyant force at least three orders of magnitude larger than the seismic background noise…” – Streckeisen †
† Streckeisen et al., “The Leaf-Spring Seismometer: Design and Performance,”Bull. Seismo. Soc. of America, vol.72, no.6, p.2352, December 1982.
gVF proofbuoy ρ= gVRTPW
F proofbuoy =
• Buoyancy force varies due to air density variations• Ideal gas law predicts air density varies with pressure
65Aaron Barzilai Stanford University
0 20 40 60 80 100 120-20-10
01020
Time [min]
Pres
sure
Var
iatio
n [P
a]Expected Pressure Variation
0 20 40 60 80 100 120-10-505
10
Time [min]
Gro
und
Vel
ocity
[um
/s]
Capacitive Geophone Guralp CMG-40T
66Aaron Barzilai Stanford University
Measured Atmospheric Pressure Variations
• Data obtained from University of Washington, Dept of Atmospheric Sciences, J. E. Tillman and Neal C. Johnson (http://www.atmos.washington.edu/~neallog/temp_real_pressure.html)
• 1 mbar = 100 Pa
67Aaron Barzilai Stanford University
What other changes are possible?
• Electron Tunneling for displacement sensing– Tunneling is a quantum mechanical effect that occurs when two
electrodes with a voltage potential between them come very closetogether
– Typically see 1nA currents when electrodes are about 10 Å apart– Current varies with gap, yielding a generic motion sensing
mechanism, similar to capacitance or inductance– Has been used in infrared sensors and accelerometers
• Velocity Feedback geophone– Use coil as both sensor and actuator– Only add circuitry, no mechanical hardware modifications
68Aaron Barzilai Stanford University
Tunneling Currenty
oeII φα−=
0
5
10
15
20
0 20 40 60 80 100
Tun
nelin
g C
urre
nt[n
A]
Electrode Gap [Angstroms]
OperatingPoint
Hz13-10 of Resolution Typical m
69Aaron Barzilai Stanford University
Tunneling Implemented
Fixed Mount
Moving Mount
MovingElectrode
Fixed Electrode
EpoxySupport
70Aaron Barzilai Stanford University
Tunneling Results
• Obtained expected sensitivity of 2.5x103 V/(m/s2) in practice when operating correctly
• Difficult to hold electrode gap steady– Sensitive to the environment as well as ground motion
• Attempted to measure work function F– Could only set bounds on the value– Greater than 10-2eV, impossible to be over 1eV
71Aaron Barzilai Stanford University
Tunneling Resolution
• Tunneling geophone sees the peak at 35 Hz
• In general, tunneling geophone’s resolution is poor
• Believe it is measuring signals due to other stimuli, such as air currents
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1 100 101 102
Frequency [Hz]
Res
olut
ion
[(m
/s)/√
Hz]
TunnelingGeophone
Guralp CMG-40T
72Aaron Barzilai Stanford University
Velocity Feedback Control Design
totalx&&
)( 2sm
Mechanical System
1200421
2 ++ ss
y
(m)Gs
posnV
(V)
InductiveVelocity Sensor
( )sKcontV
(V)
Controller
3741contI
(A)
Resistor
GcontF
(N)
Coil-Magnetas Actuator(G)
025.1
Mass
inx&&
)( 2sm
Drive Circuitry
contx&&
)( 2sm
+
-
73Aaron Barzilai Stanford University
Velocity Feedback Measured Sensitivity
• Model predicts constant sensitivity to ground acceleration out to 50 Hz
• Measured data matches well
• This approach won’t improve low frequency resolution
• Velocity Feedback won’t produce an affordable, broadband seismometer 10-2
10-1
100
101
100 101 102 103
Frequency [Hz]
Sens
itivi
ty [V
/g]
74Aaron Barzilai Stanford University
Talk Outline
• Introduction• Why isn’t a geophone an affordable, broadband seismometer?• How can a geophone be transformed into an affordable,
broadband seismometer? • How well do prototypes of a capacitive geophone perform?
– Does the measured sensitivity match predictions?– Has the low frequency resolution been improved?– What limits the low frequency resolution?
– Do the prototypes meet PEPP’s requirements?
75Aaron Barzilai Stanford University
Measured Distant Earthquakes
M7.3 Quake near Australia on 11/26/99 recorded at Stanford
54 56 58 60 62 64 66-60
-30
0
30
60
Time [min]
Mea
sure
d G
roun
d M
otio
n [u
m/s
]
Guralp CMG-40TCapacitive Geophone
76Aaron Barzilai Stanford University
0 0.5 1 1.5 2-60-30
03060
Cap
Geo
O
utpu
t [um
/s]
0 0.5 1 1.5 2-60-30
03060
Gur
alp
Out
put [
um/s
]
0 0.5 1 1.5 2-300-200-100
0100
Time [hours]
Con
v G
eoO
utpu
t [m
V]
77Aaron Barzilai Stanford University
Cost Estimate: Volumes ≈10
From Standard suppliers (eg Digikey, Allied)$65PCB Components
8 hours at Grad Student Wages$68Labor
Meets Target$453Total
Paramont Precision Inc$150Current Mechanical Hardware
AP Circuits$20Printed Circuit Board
Paramont Precision Inc$85Environmental Isolation Mechanical Hardware
Includes setup fee, OYO Geospace $65Geophone with holes
CommentCostItem
78Aaron Barzilai Stanford University
Conclusions and Contributions
• Have modified a geophone to produce a seismometer capable of measuring teleseisms
• Expected price could be less than $1,000, enabling their use in high schools
• Presented a technique for measuring noise of sensors in the presence of large background signals
• Next step: In conjunction with PEPP, develop a more professional prototype with better environmental isolation
79Aaron Barzilai Stanford University
Publications• A. Barzilai, T. VanZandt, and T. Kenny, ``Technique for measurement of the noise of a sensor in the presence of
large background signals,'' Review of Scientific Instruments, vol. 69, no. 7, pp. 2767-2772, July 1998.• C.H. Liu, A. Barzilai, J.K. Reynolds, A. Partridge, T. Kenny, J. Grade, and H. Rockstad, ``Characterization of a
high-sensitivity micromachined tunneling accelerometer with micro-g resolution,'‘ Journal of Microelectromechanical Systems, vol. 7, no. 2, pp. 235-244, July 1998.
• A. Barzilai, T. VanZandt, T. Pike, S. Manion, and T. Kenny, ``Tunneling Seismometers: A Tunneling Geophone,'' Conference Proceedings, American Society of Mechanical Engineers International Congress, Nov. 1999.
• C.H. Liu, A. Barzilai, O. Ajakaiye, H.K. Rockstad, and T.W. Kenny, ``Performance Enhancements for theMicromachined Tunneling Accelerometer", Conference Proceedings, International Conference on Solid State Sensors and Actuators, June 1999.
• A. Barzilai, T. VanZandt, T. Pike, S. Manion, and T. Kenny, ``Improving the Performance of a Geophone through Capacitive Position Sensing and Feedback,'' Conference Proceedings, American Society of Mechanical Engineers International Congress, Nov. 1998.
• C.H. Liu, J.K. Reynolds, A. Partridge, J. Grade, A. Barzilai, T. Kenny, and H. Rockstad, ``High-sensitivity micromachined accelerometer based on electron tunneling transducers,'' Conference Proceedings, American Society of Mechanical Engineers International Congress, Nov. 1997.
• J. Grade, A. Barzilai, J.K. Reynolds, C.H. Liu, A. Partridge, H. Jerman, and T. Kenny, ``Wafer-scale processing, assembly, and testing of tunneling infrared detectors'' Conference Proceedings, International Conference on Solid-State Sensors and Actuators, June 1997.
• J. Grade, A. Barzilai, J.K. Reynolds, C.H. Liu, A.Partridge, L. Miller, J. Podosek, and T. Kenny, ``Low frequency drift in tunnel sensors'' Conference Proceedings, International Conference on Solid-State Sensors and Actuators, June 1997.
• C.H. Liu, J. Grade, A. Barzilai, J.K. Reynolds, A.Partridge, H. Rockstad, and T. Kenny, ``Characterization of a high-sensitivity micromachined tunneling accelerometer'' Conference Proceedings, International Conference on Solid-State Sensors and Actuators, June 1997.
• J. Grade, A. Barzilai, J.K. Reynolds, C.H. Liu, A.Partridge, T. Kenny, T.VanZandt, L. Miller, and J. Podosek, ``Progress in tunnel sensors'‘ Conference Proceedings, Solid-State Sensor and Actuator Workshop, June 1996.
80Aaron Barzilai Stanford University
Acknowledgements• Tom Kenny• Ed Carryer, Chris Gerdes, Steve Rock, Greg Beroza• Tom VanZandt, Steve Manion, Tom Pike @ JPL (CSMT)• NSF Career Award, Charles Lee Powell Foundation,
Terman Fellowship• IRIS(Marcos Alvarez), PSN(Larry Cochrane), PEPP,
SPDL, PRL, CDR, RPL, Goodson Lab, Biomotion Lab, Design Division
• Kennylab, particularly John Grade, Jonah Harley, Cheng-Hsien Liu, Olaleye Ajakaiye, Kevin Yasumura, Eugene Chow
• Friends on and off campus• Family
81Aaron Barzilai Stanford University
Acknowledgements
Thanks, Jess
82Aaron Barzilai Stanford University
83Aaron Barzilai Stanford University
Inner Controller Transfer Function
10-5 10-4 10-3 10-2 10-1 100 101 102 10310-8
10-6
10-4
10-2
100
Frequency [Hz]
Con
trol
ler
Mag
nitu
de []
10-5 10-4 10-3 10-2 10-1 100 101 102 103-100
-80
-60
-40
-20
0
Frequency [Hz]
Con
trol
ler
Phas
e [d
eg]
84Aaron Barzilai Stanford University
Outer Controller: Circuit
2.5 M
0.1 µF
G
10 k
10 k 47.5 k
0.1 µF
4.99 k
10 k 10 k
1000 µF
10 k
10 k
374
85Aaron Barzilai Stanford University
Outer Controller: Extend Bandwidth
inx&& totalx&&
)( 2sm
Mechanical System
1200421
2 ++ ss
y
(m)
contx&&
)( 2sm
5106×posnV
(V)
CapacitivePosition Sensing
125001
+s
contV
(V)
Controller
3741contI
(A)
Resistor
30contF
(N)Coil-Magnetas Actuator
025.1
Mass
contx&&
)( 2sm 1
121
22
11
1
2
++
sCRsCR
RRcontV
(V)3741contI
(A)30
contF
(N)025.1
86Aaron Barzilai Stanford University
Position Measurement Circuitry
A
4
2
5
3
B
10 M
TLC2274
10 k
10 k
OP470
MAX4526
2.21 k
0.1 µF
OP470
A
B
C
C
4
2
87Aaron Barzilai Stanford University
Seismometer Resolution
• Capacitive Geophone has better resolution than a Conventional Geophone at low frequencies, but not at high frequencies
• Capacitive geophone is almost 2 orders of magnitude noisier than a Guralp CMG-40T
• Expect Capacitive Geophone noise can be reduced by reducing coupling to the environment….
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-3 10-2 10-1 100 101
Frequency [Hz]
Guralp CMG-40T
Res
olut
ion
[(m
/s)/√
Hz]
GeophoneThermomechanical
Limit
ConventionalGeophone
CapacitiveGeophone
88Aaron Barzilai Stanford University
More on Environmental Coupling
• According to Streckeisen, “…the sensor must be protected from variations of the atmospheric air pressure; its seismic mass would otherwise experience a variable buoyant force at least three orders of magnitude largerthan the seismic background noise…” †
† Streckeisen et al., “The Leaf-Spring Seismometer: Design and Performance,”Bull. Seismo. Soc. of America, vol.72, no.6, p.2352, December 1982.
MovingMass
mg ρVg
ky Buoyancy Force ( )?tρ
VRT
WmP =
Wmn =nRTPV =
WRTP ρ=
RTPW=ρ
89Aaron Barzilai Stanford University
More on Environmental Coupling
gVF proofbuoy ρ= gVRTPW
F proofbuoy =
KmolJ
molkg
air .RW ⋅− =×= 3148 1097.28 3
KT.gsm 300 89 2 ≈=
( )( )322223 108.5]1065.1[]102.2[]m[ −−− ××−×= πproofV.85 in .65 in .2 in
]m[109.3 36−×=proofV
AssumeConstant
]m[104.4][]N[ 210mN
2−×⋅= PFbuoy
90Aaron Barzilai Stanford University
More on Environmental Coupling
][108.1][ 22 mkgN8
mN
⋅−×⋅= Pxbuoy&&
][108.1][21
22 mkgN8
mN
⋅−×⋅= P
fxbuoy π&
91Aaron Barzilai Stanford University
Power Consumption
• Capacitive Geophone: – +7V, 27mA & -7V, 21mA ⇒ 330 mW total– 8mA each for 2 OP-470’s = 16mA– 3 mA for TLC2274– 0.5 mA each for 2 MAX4526’s = 1mA– 0.5 µA for HA7210 =0.0005 mA
• Streckeisen– 700 mW for 3-axis “Low Power” seismometer
• Guralp– 550 mW for 3-axis seismometer
92Aaron Barzilai Stanford University
Placing Geophone Mechanics Outside Seismic Vault Causes Most Noise
• Noise is largest when geophone mechanics are outside vault, electronics in
• Slightly increased noise if electronics outside vault, mechanics inside
• Have both inside as a reference• Data is repeatable• One electronics outside result
obtained after testing mechanics outside, 2 others taken before
10 -8
10 -7
10 -6
10 -5
10 -4
10 -3
10 -2 10 -1 10 0Vel
ocity
NSD
[(m
/s)/¦
Hz]
Frequency [Hz]
Datasets
Green: Mech Inside, Elec InsideBlue: Mech Inside, Elec Outside
9,10,11,1207,08,13
05
Red: Mech Outside, Elec Inside
Taken From Report MechEnvSens110999
93Aaron Barzilai Stanford University
Schematic
AdditionalHousing
MovingElectrode
FixedElectrodes
Insulation
33.37 mm39.37 mm
y
CircuitModel
a = Balanced Gap ≈ 250µm
A = Area = 3.4 ×10-4 m2
CNOMINAL = 12.1pF
C =εε0 Aa − y
C =εε0 Aa + y
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